1. Field of the Invention
This invention relates to improving the water loss properties of cement slurries during the cementing of oil wells. Such is accomplished by incorporating within the cement slurry a polymer obtained by radiation induced polymerization.
2. Description of the Prior Art
Additives have been mixed with cement slurries to reduce the water loss from the cement slurry into the subterranean formation during cementing thereof. By preventing water loss, better cementing jobs can be obtained. Extensive water loss can result in formation damage resulting from cement filtrate waters and "flash setting", the latter resulting in a cement having undesirably low strength.
Agents such as carboxymethyl cellulose and hydroxyethyl cellulose, clays such as bentonite, and like additives have been added to reduce water loss. It is especially important to prevent water loss when cementing a porous formation from which the mud cake has been removed, i.e. the cement may become quickly dehydrated and undergo a so-called "flash set". Such an adversity may cause the pipe to stick and prevent the rotation or reciprocation desirable when wall scratches are used in the hole. "Wall scratches" are used to improve the bond between the cement and the formation, the scratches actually scratching the interior of the well-bore so that the cement can more readily bond to the formation wall.
Good fluid loss control properties of the cement slurry are essential, especially to control the deposition of solids wherein a differential pressure exists between the cement slurry and the permeable zones of the formation to be cemented. Also, during the cementing of casing, if the formation has highly permeable zones, this may cause premature dehydration of the cement slurry and thus a less effective cementing job. Also, in squeeze cementing, controlled deposition facilitates filling all of the perforations without excessive squeeze pressures--if high fluid loss is realized, then of course controlled deposition will not be effected.
Other reasons exist for using radiation-induced polymers in cement slurries; for example, the slurries are easier to pump into the wellbore if they experience low water loss characteristics, and as a result, less energy is required to pump the cement slurry down into the wellbore. Also, lower pump pressures, in turn, can reduce water loss problems. Also, the slurry viscosity tends to maintain better stability if the water loss characteristics are reduced, thus enabling one to design the cementing job more effectively and efficiently.
The prior art has tried high molecular weight synthetic polymers as fluid loss control agents for cement slurries. Examples of such polymers include commercially available, partially hydrolyzed polyacrylamides.